1. Field of the Invention
The present invention relates to a solid laser amplifier, a solid laser unit and a solid laser excitation method, capable of generating a high output and high quality laser beam.
2. Description of Related Art
FIG. 8 is a side view of a conventional semiconductor-laser excited solid laser unit (hereinafter, simply referred to as solid laser) disclosed, e.g., in Japanese Patent Laid-Open No. 4-240786. At the rear of a total reflection mirror 4 on the base 8, the solid laser unit comprises a diode 1, a semiconductor 2 and a semiconductor laser emitter section 3 as shown in FIG. 8. The photodiode 1, disposed at a position close to the semiconductor laser emitter section 3, comprises three types of photodiode regions 1a, 1b and 1c having predetermined different band-pass filter characteristics. The semiconductor laser 2 is provided on a temperature controller 7 for controlling its temperature. In front of the total reflection mirror 4, the solid laser unit comprises a solid laser medium 5 and a partial reflection mirror 6. The total reflection mirror 4 is so arranged as to have a high transmittance for the excitation light 9 emitted from the semiconductor laser emitter section 3 but almost totally reflect a laser beam 10.
FIG. 9 is a graph showing the comparison of the respective band-pass filter characteristics in the photodiode regions 1a, 1b and 1c with the absorption spectrum of a solid laser medium, e.g., made of Nd:YVO4. In FIG. 9, the respective band-pass filter characteristics in the photodiode regions 1a, 1b and 1c are indicated by broken lines, whereas the absorption spectrum of the solid laser medium is indicated by a solid line. As evident from FIG. 9, the wavelength regions of three types of band-pass filters are so designed as to overlap with the absorption spectrum of the solid laser medium. To be specific, the respective wavelength regions of three types of band-pass filters are allotted to the photodiode regions 1a, 1b and 1c from the short wavelength side and the peak of the filter of the photodiode region 1b among them in the wavelength region is so designed as to coincide with the peak wavelength of the absorption spectrum of the solid laser medium.
The conventional solid laser unit is so constituted as mentioned above wherein the excitation light 9 emitted from the semiconductor laser emitter section 3 is introduced into the solid laser medium 5 to convert the solid laser medium 5 into a laser amplification medium by exciting it. The natural emission light generated from the laser amplification medium is amplified while travelling and returning between the light resonator comprising a total reflection mirror 4 and a partial reflection mirror 6 to form a directional laser beam 10, and is emitted outward as laser beam 11 when its energy reaches or exceeds a predetermined magnitude.
Setting the oscillation wavelength of the semiconductor laser 2 can be accomplished by the temperature control of the semiconductor laser 2. Thus, with the conventional solid laser unit, as disclosed in Japanese Patent Laid-Open No. 4-240786, by monitoring the currents of the photodiode regions 1a, 1b and 1c while controlling the temperature of the semiconductor laser 2 with a thermocontroller 7 and maximizing the monitor current of the photodiode region 1b in which the peak of the band-pass filter in wavelength region is so designed as to coincide with the peak wavelength of absorption spectrum of the solid laser medium 5, the wavelength peak of oscillation of the semiconductor laser 2 and the peak wavelength of the absorption spectrum of the solid laser medium 5 are made coincident with each other.
With the conventional solid laser unit, as mentioned above, by an arrangement of setting the oscillation wavelength of the peak of the semiconductor laser in such a manner as to coincide with the peak wavelength of absorption spectrum of the solid laser medium 5, the excitation efficiency of the solid laser medium 5 is elevated.
With such an arrangement, however, excitation by a large power semiconductor laser to obtain a high output would lead to a strong excitation near the end face of the solid laser medium 5, thereby causing a phenomenon that the intensity distribution of light in the solid laser medium becomes nonuniform and the shape of a laser beam is broken. Consequently, the conventional arrangement had a problem that a high power and high quality beam could not be generated even though a higher excitation efficiency was possible.
Furthermore, since the monitor currents from three photo diodes 1a, 1b and 1c were compared and the thermocontrol of the semiconductor laser 2 was performed in such a manner that the monitor current of the photo diode 1b is maximized as mentioned above, the conventional solid laser unit also had problems that the need for constructing a complicated thermocontrol system did not only complicate the unit arrangement but raised the cost.